Attenuation network



Dec. 19, 1939.

J. P. SMITH. JR

ATTENUATION NETWORK Original Filed Aug. 28, 1937 2 Sheets-Sheet 1 /7 INVE TOR. Cf/H BY RNEYS.

Patented Dec. 19, 1939 UNI sn sri'ss ATTENUATION NETWORK John P. Smith, Jr., Ridgewood, N. J., assignor to The Daven Company, Newark, N. J., a crpora-.

tion of New Jersey Application August 28, 1937, Serial No. 161,502 Renewed June 15, 1939 6 Claims.

This invention. relates to variable attenuation networks.

In a variable attenuation network of the bridged T type, two switch arms are simultaneously moved over two sets of contacts connected to two sets of series-connected resistance units to vary the resistance in the arms of the network while maintaining the impedance substantially constant, the resistance units being so designed that the attenuation is varied in equal increments. In order to keep the overall impedance constant, the individual resistance units must be wound within tolerances of 2%. It is extremely diflicult to wind resistance units of lower value than one ohm within such limits in production and furthermore resistance units about 5,000 ohms are diificult to wind non-inductively unless separate spools are used. This increases notonly the cost of manufacture, but also the size of the attenuator and slows down production.

An object of this invention is an attenuation network in which the use of extremely small and extremely high resistance units is obviated thereby facilitating its manufacture and minimizing its cost. I

In an attenuation network embodying the invention, a pair of series connected resistors'of fixed value are bridged by a variable resistance and a Variable shunt resistance is connected between the first-named resistors. For some high impedance networks the arrangement is such that the attenuator is controlled at the higher attenuation positions by keeping the bridged resistance constant and Varying only the shunt element while for low impedance networks, this arrangement is reversed, that is the shunt element is kept constant while the bridged resistance is varied. In this latter condition, the attenuator is converted from a bridged T type to a 1r type at the higher attenuation positions. In the case of a high impedance network at high attenuation steps, the resistance values in the shunt arm are designed to control the network loss variation while the bridge arm is kept constant and in the case of low impedance network at the higher attenuation settings, the resistance values of the bridged arm are designed to control the network loss variation while the shunt arm is kept constant.

In another type of high impedance network, it is possible to maintain the nominal input and output impedance within limits of after a total of 30 decibels attenuation has been reached, by first Varying the bridge arm for a few steps while holding the shunt element constant and then opening the bridge arm entirely and varying the shunt arm in logarithmic fashion similar to the variations in a potentiometer. In all types the change in the electrical circuit of the network itself, is permissible only in the higher attenuation values. It is a known fact that a certain amount oi attenuation will. completely shield impedance variations even of extreme values from being measured thru the attenuator.

In these cases, the impedance variations which take place by keeping one element constant while varying the other, occur within the network itself and inasmuch as the attenuation of the network is approximately 30 db. in both directions, the impedance variations which are presented to the attenuator terminals are within commercial limits. Actually, the network is changed from a bridged T to a ir whenever the shunt resistance is kept constant while the bridge element is varied.

Other objects, novel features and advantages of this invention will become apparent from the following specification and accompanying drawings, wherein:

Fig. 1 illustrates diagrammatically a high impedance network embodying the invention;

Fig. 2 illustrates diagrammatically a low impedance network embodying the invention; and

Fig. 3 illustrates diagrammatically another high impedance network embodying the invention.

In Fig. 1, 30, i I and 52 are the input, output and common terminals respectively. A resistor i3 is connected in series with a resistor M- between the binding posts Hi and ii. A resistance it has one end connected between the binding post and the resistor is and is composed of a plurality of units for which are provided the contacts C-il to (3-16. Additional contacts C-il', C-iil and Ci9 are strapped to the contact C-iii. A switch'arm iii is connected between the resistor 5 and the output terminal H and is designed to engage the contacts C-2l to C-iil and also a Contact C-iil which is arranged after the contact 6-H) and is unconnected to the resistance It. A resistance ii is connected at one end to the binding and is composed of a plurality of units for which are provided contacts C--i to C--2il. A switch arm i8 is designed to engage the contact which is arranged ahead of the contact Ci but is unconnected to the resistance ii and is signed to engage the contacts Ci to C--i-2ii, this switch arm being interconnected the switch arm it for simultaneous movement therewith so that the two switch arms at all times engage corresponding contacts of the resistances i5 and ll.

Zero attenuation is obtained with the switch arms l6 and I8 respectively engaging the contacts -D and C'-0 in which condition the resistance i is short-circuited and the resistance It is opened. When the switch arms l5 and it engage contacts CI and C'l respectively, the first increment of attenuation is obtained and with this arrangement the network is of the bridge T type. Attenuation is increased by moving the switch arms l6 and ill over the contacts and after the switch arm 55 engages the contact 0-55, the network is controlled by the variations in resistance obtained in the shunt arm only. From contact C-l6 to (7-40, the condition of a bridge .T type attenuator is maintained.

but with no change in the value or" the bridge fl resistance. The attenuator variation for the last three steps is efiected solely by changing the resistance value in the shunt arm. The high value esistance units which would have been required hr a standard bridge T type attenuator are thus obviated while the higher attenuator resistance units in the shunt arm all exceed one ohm in value.

In the attenuator just described, the resistors it and M are of rather high impedance, for example, 500 ohms and with resistors of this value, the resistances I50, I51) and Hid are respectively approximately 2583 ohms, 3220 ohms and i100 ohms, while the resistances [16, EM, :10, iii) and Na are respectively 4.40 ohms, 3.45 ohms, 5.63 ohms, 4.19 ohms and 3.06 ohms. With the standard bridge type attenuator, the resistance units between contacts C-IG to Cl9 respectively will be of much higher value than Ma and the avoidance of these resistances simplifies the manufacture of the attenuator and reduce not only its cost but also its size.

In Fig. 2, In, H and [2 again indicate the input, output and common terminals respectively. Also, l3 and I4 indicate resistors connected in series between the terminals I0 and I l. A resistance l5 has one end connected between the input terminal l0 and the resistor [3. This resistance is composed of a plurality of units for which are provided contacts C-D to Cl8. A pair of open contacts 0-49 and (3-20 are provided after the contacts Cl8. A switch arm l5 adapted for engagement with the contacts C-t to (3-20 is connected between the resistor i l and the terminal I l. A resistance i1 is connected at one end to the terminal l2 and is composed of a plurality of units for which are provided contacts C'-l to C'I5 inclusive, C'-I9 and C-ll3. Contacts C-|l and Cl8 are strapped between contact C'l6 and contact 0-49. A switch arm l8 designed to engage the contacts C'!! to C'-20 is connected between the resistors l3 and I4 and is mechanically interconnected to the switch arm I6 for simultaneous movement therewith so that the two switch arms at all times engage corresponding contacts.

Zero attenuation is obtained with the switch arms l0 and I8 respectively engaging the contacts C-0 and CB and the attenuation is increased by moving the switch arms over the contacts, the first increment of attenuation being effected with the switch arms engaging the contacts C-l and C'l respectively. The network remains of the bridge T type until the switch arms are brought into engagement respectively with the contacts C-l6 and C'l6, at which point the network is contacted into a 1r type attenuator in which the resistance l5 forms the top and the resistances l3 and hi form the legs in series with that portion of the resistance I'l lying between the contacts Cl9 and C'2fl. This condition is maintained while the switch arms are brought successively into engagement with the contacts C-l5 to 0-H and Cl5 to Cl9. The small resistance units which would be required in a bridge T type network between the contacts C-l6 and Ci9 are obviated and the attenuator is compensated for by use of larger resistance units between contacts 0-46 to C-I9 than would be required in the bridge resistance of a bridge I type network.

In the attenuator of Fig. 2, the resistors I3 and I4 are of low impedance, for example, 15 ohms. With resistors of this value, the units 15f, 5c and 15d respectively are approximately 122 ohms, 495 ohms and 1985 olnns, while the resistance unit H is approximately 3.87 ohms. The resistances i501, We and l5f while comparatively large are still considerably less than 5,000 ohms and are, therefore, easy to produce and of low manufacturing cost while the resistance ll'f is of such. size as also to present no manufacturing difiiculty.

Fig. 3 discloses an embodiment of the invention in which the resistors l3 and [4 are of higher impedance, for example, 600 ohms. The resistance 55 consists of a series of 17 resistance units for which are provided contacts C0 to C-ll. Beyond contact 0-41 are provided open contacts 0-48 to C30. The resistance ll consists of a series of 14 resistance units for which are provided contacts C-8 to C'l5 and a second series of 13 resistance units for which are provided contacts C"-l8 to 0-40, there being contacts C'l6 and Cll strapped between contacts C-l5 and C-I8. Switch arms I6 and i8 are provided as in the other modifications for simultaneously engaging corresponding contacts. Minimum attenuation is obtained with the switch arms 15 and I8 respectively engaging the contacts C0 and C0. Upon to step l5 there is no change from a standard bridge T attenuator. For the next three steps the network is in 1r form with the shunt resistance constant and the series arm varying in much larger value than would be required in a normal bridge T network. From step ll to step 30, the bridged element is open so that 13 resistance windings are obviated and attenuation is controlled by adjustment of the shunt arm only. The units in the resistance H from contact 0-48 to the terminal 82 form a logarithmic variation equivalent to potentiometer design to maintain equal increments in decibels. These units are of larger value than would be required in a standard bridge T and are therefore easier to manufacture. All told 16 resistors are obviated by the arrangement just described which permits the manufacture of a heretofore physically impossible thirtystep attenuator having the characteristics of a T network. When the attenuation is controlled by the shunt element, that is from step l8 to step 30, the two 600 ohm resistors form the series element of the attenuator and the resistance of the shunt element is not over 20 ohms. Under this condition, the impedance presented to the input or output terminals cannot exceed the commercial tolerance of 5%. In a standard bridge T, the bridged resistances for steps iii to St! would increase rapidly to approximately Inegohm while the shunt resistors would be less than one-half ohm for the last six steps. The resistance units i5 3, liih, and 15g respectively are approximately 3,919 ohms, 11,626 Ohms and 31,000 ohms while the resistors I'lg to Us vary from 4.06 ohms to .235 ohm, all of which present no particular manufacturing problems.

In the attenuator illustrated in Figs. 1 and 2, engagement of the switch arms it and 18 with the contacts C2il and C-2il opens the resistance l5 and short-circuits the resistance ll and the same condition exists in the attenuator of Fig. 3 when the switch arms it and i8 respectively engage the contact C3ii and C-3ii. The attenuator of Fig. 2 becomes a straight T type when the switch arms it and i8 engage the contacts 0-49 and C-l while the attenuator of Fig. 3 becomes a straight T type when the switch arms It and it engage any of the contacts from ClB and C'l8 to C-3ii and C"3!i.

I claim:

1. In an attenuation network, a pair of fixed resistances connected in series, a third and a fourth resistance, an equal number of contacts associated with each of said third and fourth resistances, certain contacts of one of said third and fourth resistances being strapped together, and a pair of interconnected switch arms adapted to simultaneously engage corresponding contacts, said third resistance and its associated switch arm being bridged across said fixed resistances and the contact switch arm for said fourth resistance being connected between said fixed resistances.

2. In an attenuation network, a pair of fixed resistances connected in series, a third and a fourth resistance, an equal number of contacts associated with each of said third and fourth resistances, certain contacts of said third resistance being strapped together, and a pair of inter connected switch arms adapted to simultaneously engage corresponding contacts, said third resistance and its associated switch arm being bridged across said fixed resistances and the contact switch arm for said fourth resistance being connected between said fixed resistances.

3. In an attenuation network, a pair of fixed resistances connected in series, a third and a fourth resistance, an equal number of contacts associated with each of said third and fourth resistances, a plurality of contacts at one end of said third resistance being open and certain intermediate contacts of said fourth resistance being strapped together, and a pair of interconnected switch arms adapted to simultaneously engage corresponding contacts, said third resistance and its associated switch arm being bridged across said fixed resistances and the contact switch arm for said fourth resistance being connected between said fixed resistances.

4. In an attenuation network, a pair of fixed resistances connected in series, a third and a fourth resistance, contacts associated with each of said third and fourth resistances, a pair of switch arms interconnected for simultaneous movement to engage corresponding contacts of said third and fourth resistances, said third resistance and its associated switch arm being bridged across said fixed resistances and the contact switch arm for said fourth resistance being connected between said fixed resistances, said fourth resistance having a greater number of units than said third resistance whereby in certain high attenuation positions of said switch arms said third resistance is open, and said fourth resistance having zero resistance between certain intermediate contacts.

5. In an attenuation network, a pair of fixed resistances connected in series, a third and a fourth resistance, an equal number of contacts associated with each of said third and fourth resistances, certain contacts at the high attenuation end of one of said third and fourth resistances being strapped together, and a pair of interconnected switch arms adapted to simultaneously engage corresponding contacts, said third resistance and its associated switch arm being bridged across said fixed resistances and the contact switch arm for said fourth resistance being connected between said fixed resistances.

6. In an attenuation network, a pair of fixed resistances connected in series, a third and a fourth resistance, an equal number of contacts associated with each of said third and fourth resistances, certain contacts at the high attenuation end of said third resistance being strapped together, and a pair of interconnected switch arms adapted to simultaneously engage corresponding contacts, said third resistance and its associated switch arm being bridged across said fixed resistances and the contact switch arm for said fourth resistance being connected between said fixed resistances.

JOHN P. SMITH, JR. 

